Device for detecting a body falling into a swimming pool

ABSTRACT

Device to deliver an alarm signal upon detection of a gravitational wave generated by a body falling into a swimming pool, using a differential detector that includes a comparison device for comparing a sensitivity threshold value to the value of the electrical signal received, and to deliver an alarm signal when the received electrical signal exceeds the sensitivity threshold value. The electrical signal resulting from the detected waves is delivered to a comparator and allows a programmed microprocessor to deliver variable-width pulses to the input of the comparator so as to reduce the sensitivity of the device when the device detects an atmospheric disturbance. The microprocessor triggers the alarm when the width of the output pulses from the comparator is larger than a predetermined critical reference and when the frequency F of the analogue electrical signal lies between two predetermined values F 1  and F 2.

TECHNICAL FIELD

The present invention relates to the detection of shock waves in theaquatic medium and relates in particular to a device for detecting abody, such as that of a child, falling into a swimming pool.

PRIOR ART

Many villas now have a swimming pool, mainly in southern regions. Theseswimming pools are generally not surrounded by safety barriers. There istherefore a high risk of an unsupervised young child walking close tothe edge falling into the water and drowning. Child deaths by fallinginto a pool currently represent one quarter of the infant mortalitycaused by accidents.

It has therefore been conceived to install detectors that detect aquaticwaves on the surface of the water in swimming pools. Such a detector isactuated when the pool is not attentively supervised, in order to beable to raise the alarm in the event of a child unluckily falling intothe pool. Unfortunately, the multiplicity of causes that result in waveson the surface of the water, which would make this type of apparatusreact, makes their use uncertain or even ineffectual owing to spuriouselements that cannot be easily controlled, especially disturbances dueto bad weather (wind or rain) that cause the alarm to be inopportunelytriggered.

A device for detecting a body falling into a pool, especially that of ayoung child, has been described in patent application No. 2,763,684.Such a device comprises a means of converting the aquatic waves pickedup by a sensing means into an electrical signal and a differentialdetector that includes a comparison means for comparing the value of asensitivity threshold to the value of the electrical signal and todeliver an alarm signal when the electrical signal results from theconversion of a gravitational wave generated by a body falling into thepool.

The differential detector used in such a device includes a sensitivitythreshold permanently set to its optimum value by the electrical signalgenerated by the sensing means, which depends on the disturbancescreated on the surface of the pool by atmospheric disturbances, such asthose induced by bad weather or a disturbance brought about by theregeneration of the water in the pool.

Such a differential detector is disclosed in patent application PCT WO01/088870. It includes autoregulation means consisting mainly of ananalogue/digital converter, the input of which is connected to theoutput of an amplifier, the input of which is connected to the output ofthe sensing of the aquatic waves in order to deliver, as output, adigital signal according to the disturbance. A programmed microprocessordelivers, in response to the detection of the digital signal deliveredby the converter, a digital signal to the “−” input of the comparator,the pulses of which have a variable width that increases with theduration and with the magnitude of the disturbance so as toautomatically increase the trigger threshold of the alarm device andtherefore to reduce its sensitivity when the acoustic sensor detects anatmospheric disturbance, such as wind, or a disturbance due to thesystem for regenerating the water in the pool.

Such a device operates perfectly well when the disturbance detected atthe input reaches its optimum phase in a steady manner. Unfortunately,when the pool filtration system is switched on (most of the timesuddenly), or when the atmospheric disturbance occurs suddenly, thedevice does not have time to increase its sensitivity threshold beforethe alarm system is inopportunely triggered.

Furthermore, a device for detecting a child falling into a swimming poolmust be entirely reliable, that is to say it must detect this fall withcertainty. It is therefore necessary for such a device to recognize,unequivocally, that is to say with 100% reliability, the “signature”caused by a child falling into the pool.

SUMMARY OF THE INVENTION

It is for this reason that the object of the invention is to provide adevice for detecting a child falling into a swimming pool that canrecognize this fall unequivocally while continually carrying out itsautoregulation function so as to avoid any inopportune triggering.

The subject of the invention is therefore a device intended to deliveran alarm signal upon detection of a gravitational wave generated by abody falling into a swimming pool, which comprises a means of sensingaquatic waves that is placed beneath the surface of the water of theswimming pool, a means of converting the aquatic waves sensed by thesensing means into an analogue electrical signal, and a differentialdetector that includes comparison means for comparing the sensitivitythreshold value of the differential detector with the value of theanalogue electrical signal and to deliver the alarm signal when theanalogue electrical signal exceeds the sensitivity threshold value. Thedifferential detector comprises autoregulation means consisting mainlyof an analogue/digital converter that receives the preamplified analogueelectrical signal as input and delivers a digital signal as output whena disturbance in the water occurs, a comparator, the “+” input of whichreceives the preamplified analogue electrical signal, and amicroprocessor programmed to deliver, in response to the detection ofthe digital signal delivered by the converter, a digital signal to the“−” input of the comparator, the output pulses of which have a variablewidth, which increases with the duration and with the magnitude of thedisturbance so as to automatically increase the threshold for trippingan alarm means and therefore to reduce the sensitivity of the devicewhen the sensing means detects an atmospheric disturbance, such as wind.The device is characterized in that the microprocessor triggers thealarm means when the width of the output pulses from the comparator islarger than a predetermined critical reference and that the frequency Fof the analogue electrical signal lies between two predetermined valuesF1 and F2.

BRIEF DESCRIPTION OF THE FIGURES

The aims, objects and features of the invention will become more clearlyapparent on reading the description that follows, with reference to thedrawings in which:

FIG. 1 is a schematic of a device according to the invention fordetecting a body falling into a swimming pool,

FIG. 2 is a block diagram of a device according to the invention,showing all the components of the differential detector,

FIG. 3 represents a time plots of the input and output signals of thefirst comparator used in the device according to the invention,

FIG. 4 represents a time plots of the input and output signals of thesecond comparator used in the device according to the invention,

FIG. 5 is a flowchart or the autoregulation procedure used in the deviceaccording to the invention,

FIG. 6 is a flowchart for the autocalibration phase used in the deviceaccording to the invention,

FIG. 7 shows the time plot of the amplitude of the aquatic waves causedby a child falling into a pool, and

FIG. 8 shows the plot of the frequency of the aquatic waves caused by achild falling into a pool as a function of the distance between theimpact and the detector.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment of the invention illustrated in FIG.1, the device comprises a right-angled tube 10, the vertical portion ofwhich is immersed in the water so that the inlet of the tube is a fewcentimeters below the surface of the water in the pool. The tube isconnected at its external end to a chamber 12 in which there is amicrophone 13 connected to a differential detector 14. The latter isconnected to an alarm means 16, such as a buzzer or a siren, or anyother indicating device, via a switch 18 for disconnecting the alarmmeans when the pool is supervised.

The water level inside the tube 10 is normally stable. However, anychange in this level causes a variation in the pressure of the air inthe tube and in the chamber 12, and thus gives rise to the emission ofacoustic waves that are converted by the microphone 13 into anelectrical signal.

The gravitational wave generated by a body (such as that of a youngchild) falling into the water in the pool essentially propagates belowthe surface of the water. Even though it is visually barely perceptibleon the surface, it causes a sudden variation in the level inside thesubmerged tube owing to the upward vertical thrust. A sudden variationin this level by a few millimeters is therefore interpreted by thedifferential detector as a signal that triggers the alarm.

However, any turbulence created on the surface by the weather and thehorizontal current brought about by the regeneration of the water causevariations in the level inside the submerged tube. These variations aresensed by the differential detector, but their low amplitude actuatesthe autoregulation mechanism, which prevents the alarm from beinginopportunely triggered.

In the embodiment illustrated in FIG. 1, the part out of the water ispreferably a sealed plastic case containing a battery for supplying thedetector, it being possible for this battery to be kept charged by asolar sensor serving as cover for the case.

Apart from the microphone 13 responsible for picking up the acousticsignals and the alarm means 16, the device according to the inventionmainly consists of the differential detector that is illustrated in FIG.2.

The signals coming from the microphone 13 are transmitted, on the onehand, to the “+” input of a constant-gain amplifying means 20 and, onthe other hand, to the “+” input of a variable-gain amplifying means 22via a resistor 24 connected to a voltage of 0.8 volts.

The amplifying means 20 is mainly composed of an operational amplifier26 that has, between its “−” input and its output, a resistor (with avalue of 3.3 MΩ) and a capacitor (with a value of 1 nF) serving asfeedback for limiting the gain. The “−” input is grounded via anelectrolytic capacitor 28 preventing the quiescent voltage from beingamplified.

The amplifying means 22 is mainly composed of an operational amplifier30 that has, between its “−” input and its output, a resistor (with avalue of 4.7 MΩ) and a capacitor (with a value of 1 nF) serving asfeedback for limiting the gain. The “−” input is grounded via anelectrolytic capacitor 32, which prevents the quiescent voltage frombeing amplified, and via a 210 to 10 000 potentiometer 34, which isadjusted according to the room in which the alarm device is installed,the necessary gain of the amplifying means being lower the moresoundproof said room is.

The output (signal 51) from the amplifying means 20 is sent to the “+”input of a comparator 36, the function of which is to convert theanalogue signal delivered by the amplifying means 20 into a binarysignal, the width of which depends on the magnitude of the disturbance,said binary signal being transmitted to the microprocessor 38 for thepurpose of autoregulating the alarm device.

In fact, when an atmospheric disturbance, such as wind, occurs, thisdisturbance generates a modulated signal at the output of the amplifyingmeans 20, such a signal generally having a low frequency of between 10and 20 Hz. This signal, delivered to the “1” input of the comparator 36,results in a digital output signal (signal S2) at the output 40 of saidcomparator and therefore at the input of the microprocessor 38. Thelatter, which detects a value 1 at the output 40 of the comparator 36,then transmits, after a given delay, digital pulses on the output line42, the purpose of which is to reduce the sensitivity of the device soas not to trigger the alarm inopportunely in the event of the windblowing, as will be seen below.

The output of the amplifying means 22 is connected to the “+” input of acomparator 44, which converts the analogue signal delivered by theamplifying means 22 into a binary signal (signal S4) that is transmittedto the microprocessor 38. When a signal corresponding to a child fallinginto the pool is recognized by the microprocessor 38, the lattertransmits a signal to the alarm means 16, which could be a radiotransmitter transmitting the alarm signal to an alarm room.

As was seen above, the microprocessor 38 is programmed to transmit asignal on its output 42 when it detects, on its input 40, a digitalsignal of value 1 coming from the comparator 36. This signal is formedfrom negative pulses of variable width depending on the number and onthe width of the pulses of value 1 that are detected on the input 40.Consequently, assuming that this input is sampled at a frequency of 150Hz, an input bit with a frequency of 15 Hz will therefore be sampledabout 5 times if the received signal is a perfect sinusoid. At eachsampling, the width of the pulse transmitted on the line 42 will beincreased. Likewise, this width is reduced each time that themicroprocessor detects the value 0 of the signal on the line 40. It maytherefore be seen that the stronger the wind, the wider the transmittedpulses output by the comparator 36 and also the wider the negativepulses delivered on the line 42. Pulse width modulation is thusobtained.

The negative pulses transmitted on the line 42 charge up, via theresistor 48 (having a value of 4.7 MΩ), the capacitor 46 (having a valueof 1 μF) to a greater or lesser extent, thereby delivering a voltagewhose value depends on the width of the pulses delivered on the line 42.The wider these pulses, the less the capacitor 46 is charged, and thehigher the voltage signal (S3) delivered on the “−” input of thecomparator 44, the lower the sensitivity of the comparator 44 reactingto the signal received from the sensor 13 in order to trigger the alarm16. It should be noted that the time during which the microprocessor 38is reacting to the presence of the atmospheric disturbance, bytransmitting negative pulses of greater and greater width to theintegrator 46–48, may be limited to a maximum value such as 10 or 20 s.

With the autoregulation of the sensitivity threshold that has just bedescribed, it may therefore be seen that if the wind changes to a storm,the alarm is not triggered owing to the fact that the sensitivitythreshold of the comparator 34 has been automatically increasedbeforehand.

As will be seen in the rest of the description, the device includes atime counter R 50 used by the microprocessor during the autoregulationprocess and a time counter C 52 used by the microprocessor during aphase in which the device is periodically autocalibrated. Furthermore,there is also an analyzer 54, that analyzes the frequency F of thesignal received by the device and used by the microprocessor to triggerthe alarm.

Assuming that the signal S1 transmitted by the amplifier 26 is thesinusoidal signal as shown in the first plot in FIG. 3, the input of theamplifier 36 acts as a threshold for obtaining a pulse S2 of width TS2,illustrated on the second plot in FIG. 3. As will be seen, this pulse istaken into account by the microprocessor 38 only if its width exceeds afirst minimum reference REF1 so as to reduce the maximum sensitivity,this being done so as to prevent the device from being triggered withoutany reason, due to errors associated with the manufacturing constraintsand with thermal variations.

Assuming that the signal output by the amplifier 30 is the sinusoidalsignal shown on the first plot in FIG. 4, it is subjected to twothresholds corresponding to two values of the signal S3 at the terminalof the capacitor 32, which signal values make it possible to obtain thepulses illustrated on the second and the third plots in FIG. 4,respectively. The first threshold is a threshold for obtaining a valueREF3 below which the pulse width TS4 obtained at the output of thecomparator 44 is ignored. The second threshold is used to obtain a pulsewidth reference REF above which an analysis of the frequency 1/T of thewaves received by the device is carried out and the alarm is triggeredif this frequency lies between two predetermined values, as will be seenlater.

The autoregulation procedure according to the invention is illustratedin FIG. 5. Firstly, at the start of the procedure, the microprocessorchecks whether the counter C has completed its decrementation down to 0(or its incrementation up to a maximum value), in which case its logicvalue is equal to 1 (step 60). If this is the case, the autocalibrationphase (B) is initiated after the resetting of the counter C (that is tosay the counter resumes decrementing or incrementing), theincrementation of a variable N to N+7, N being the time during which thecapacitor 46 is being charged by the microprocessor, and the resettingof a variable OK, which will be set to 1 when the autocalibration willhave taken place (step 61). Otherwise, the microprocessor checks whetherthe counter R has completed is decrementation down to 0 (or itsincrementation up to a maximum value), in which case its logic value is1 (step 62).

If the counter R has already reached its optimum value (its logic valueis 1), a variable NS defining the sensitivity level of the device isdecremented by 1 and the counter R is again actuated (it logic value is0) (step 64). The decrementation by 1 corresponds to an increase in thesensitivity of the device. It should be noted that the sensitivity levelNS could vary from the value 0 (maximum sensitivity) to 40 (minimumsensitivity). It should also be noted that a decrementation of NScorresponds to a lowering of the threshold 1 of the signal S4 (see FIG.4).

Whether or not the variable NS has been decremented after theverification of the counter R by the microprocessor, the latterdetermines if the signal S4 is equal to 0 (step 66). If this is thecase, the microprocessor determines whether the signal S2 is also equalto 0 (step 66). If this is the case, the procedure is looped back to itsstarting point, without resetting the counter R.

If the value of S2 is not equal to 0, the microprocessor determineswhether the width TS2 of the pulse S2 (see FIG. 3) is below REF1 (step70). If this is the case, the procedure is looped back to its startingpoint, after the counters R and C have been reset (step 72).

When the value of S4 is equal to zero, the microprocessor determineswhether the width TS4 of the pulse S4 is between the reference valuesREF2 and REF (step 74). If this is not the case, the microprocessorchecks whether the value TS4 is below the lower reference REF2 (step 76)below which the disturbance signal in question is not considered asbeing significant. If this is the case, no action is undertaken and theprocedure is looped back to its starting point after the counters R andC have been reset (step 72).

When the value of TS4 is not below REF2, that is to say when it is aboveREF, this means that the signal received by the device may be caused bya body falling into the water, as explained below. The microprocessorthen checks whether the frequency F of the received signal lies betweentwo limit values F1 and F2 (step 78). If so, this means that the signalresults from a child's body falling into the pool, as explained below,and the alarm is triggered (step 80).

When S4 is equal to zero and TS2 is greater than REF1, or S4 is equal tozero and TS4 is between REF2 and REF, or S3 is equal to zero and TS4 isgreater than REF, while the frequency of the received signals does notlie between the two predetermined values F1 and F2, the sensitivityvalue NS is incremented by 2 (step 82). Such an incrementation allowsthe sensitivity threshold to be raised, although it could be reduced byone unit when the counter R has already reached 0 or its maximumcapacitance (step 64). After this incrementation, the procedure islooped back to its starting point after the counters R and C have beenreset (step 72). The purpose of resetting the counter R after eachincrementation of NS is to avoid increasing the sensitivity of thedevice too rapidly.

As has just been seen, the triggering of the alarm is subordinated tothe detection of a specified frequency of the aquatic waves received bythe detector, the determination of this frequency constituting anessential feature of the invention. This is because it has been foundthat the speed of propagation of the aquatic waves over the surface ofthe water, and therefore their frequency, depends on the volume of waterdisplaced and therefore on the volume and the weight of the body fallinginto the water, and also on the height of the fall. Insofar as, for achild, this height is approximately constant, i.e. 10 to 20 cm relativeto the surface of the water, this height will not be taken intoconsideration.

In fact, it has been found that, for a given height of fall, thefrequency of the aquatic waves depends directly on the ratio between theweight and the volume of the body that has fallen in, that is to say onits density.

Thus, a stone, with a density of 3, falling in produces aquatic waveswith a frequency of about 0.6 Hz, whereas a ball, with a density of 0.3,falling in produces waves with a frequency of about 2 Hz. In the case ofa child, having a density in the region of 1, the frequency of theaquatic waves is between 0.8 Hz and 1.2 Hz depending on the distancebetween the point of impact and the detector.

If we consider a distance of 5 m between the point of fall by the childand the detector, the train of aquatic waves (in general 4 waves)received by the detector is shown in the plot in FIG. 7. It may be seenthat the first wave (or aquatic wave) reaches the detector after about 6s and that the three other waves of the wave train arrive at decreasingintervals T₁, T₂ and T₃, the mean being about 1.12 s, i.e. a meanfrequency of about 0.9 Hz.

The frequency of the waves detected by the detector depends in fact onthe distance, as shown by the plot in FIG. 8. The greater this distance,the higher the frequency of the waves. Thus, if the distance goes from 5m to 9 m, the frequency of the aquatic waves goes from about 0.9 Hz toabout 1.15 Hz along a logarithmic-type curve. It should be noted thatthis distance must not be too great, insofar as the greater thisdistance, the longer the delay in detecting the fall. As a general rule,the detection delay could not exceed 10 s.

As was mentioned, the counter C is reset after each incident, that is tosay when S2 and/or S4 is not equal to zero. However, if no incident isdetected over a specified time, for example 15 minutes, themicroprocessor assumes autocalibration, since the value of the counter Cis equal to 1 (see step 60). Before the actual phase of autocalibratingthe device illustrated in FIG. 6, the microprocessor will have carriedout the “guard dog” test (not shown) and the initialization is carriedout, if it is the first time there is autocalibration. Thisinitialization consists in setting a variable TX to 90, which representsthe time in seconds after which the autocalibration can be carried out,in setting the variable N to zero, N representing the time over whichthe capacitor 46 is charged by the microprocessor, and in setting thelogic variable OK to zero, which variable will be reset to 1 when theautocalibration has taken place (step 84).

During the entire autocalibration phase, the first step consists inchecking whether the variable OK is equal to zero (step 86). If this isnot the case, the program returns to the main autoregulation procedure A(see FIG. 5). If the variable OK is equal to 0, the microprocessor waitsuntil the time TX has elapsed before continuing its execution (step 88).At the end of the time TX, it determines whether the value of S2 isequal to 0 (step 90). If this is the case, it determines whether thevalue of S4 is equal to 0 (step 92). If this is also the case, the valueof N is assigned a constant N₀ that indicates the reference time forcharging the capacitor 46, allowing the maximum threshold to be obtainedat the “−” input of the comparator 44, the time TX is set to 5 s, andthe variable N is incremented by 1 (step 94). The program is then loopedback to the TX waiting step (step 88). It may therefore be seen that thecapacitor charging time N is incremented every 5 s and therefore thesensitivity threshold is lowered, as no incident has occurred.

As soon as the value of S4 reaches 1 (the input S3 becomes less than the“+” input of the comparator), meaning that the limit value has beenreached, the microprocessor decrements the charging time N by 5 s sothat the “−” input is substantially lower than the “+” input, theconstant N₀ is set to N, which thus becomes the new reference value, andthe variable OK is set to 1, in order to indicate that theautocalibration phase has been completed (step 96). The program is thenlooped back to its starting point.

When the microprocessor determines that the value of S2 is not equal to0, meaning that there has probably been a disturbance, the waiting timeTX is reset to 5 s and the variable N is set to the reference value N₀(step 98). The program is then looped back to its starting point.

1. A device intended to deliver an alarm signal upon detection of agravitational wave generated by a body falling into a swimming pool,which comprises a means of sensing aquatic waves that is placed beneaththe surface of the water of the swimming pool, a means of converting theaquatic waves sensed by said sensing means into an analogue electricalsignal (S1), and a differential detector that includes comparison meansfor comparing a sensitivity threshold value of said differentialdetector with the value of said analogue electrical signal and todeliver said alarm signal when said analogue electrical signal exceedssaid sensitivity threshold value, said differential detector comprisingautoregulation means which includes an analogue/digital converter thatreceives a preamplified analogue electrical signal as input and deliversa digital signal (S2) as output when a disturbance in the water occurs,a comparator, the positive (+) input of which receives said preamplifiedanalogue electrical signal, and a microprocessor programmed to deliver,in response to the detection of said digital signal delivered by saidconverter, a digital signal (S3) to the negative (−) input of saidcomparator, the output pulses (S4) of which have a variable width, whichincreases with the duration and with the magnitude of said disturbanceso as to automatically increase the threshold for tripping an alarmmeans and therefore to reduce the sensitivity of the device when saidsensing means detects an atmospheric disturbance; wherein said devicemicroprocessor triggers said alarm means when the width (TS4) of theoutput pulses (S4) from said comparator is larger than a predeterminedcritical reference (REF) and wherein the frequency F of said analogueelectrical signal lies between two predetermined values F1 and F2. 2.The device of claim 1, in which said microprocessor assigns asensitivity level (NS) to the device, said sensitivity level beingincremented by 2 when the frequency F of said analogue electrical signaldoes not lie between said predetermined values F1 and F2 when the width(TS4) of the output pulses (S4) from said comparator is larger than saidpredetermined critical reference (REF).
 3. The device of claim 2, inwhich said sensitivity level is incremented by 2 by said microprocessorwhen the width (TS4) of the output pulses (S4) from said comparator liesbetween a second predetermined minimum reference (REF2) and saidpredetermined critical reference (REF).
 4. The device of claim 2, inwhich said sensitivity level is incremented by 2 by said microprocessorwhen the value of the output pulses (S4) from said comparator is equalto 0, while the value of the digital signal (S2) output by saidanalogue/digital converter is not equal to 0 and when the width (TS2) ofsaid digital signal is smaller than a first predetermined minimumreference (REF1).
 5. The device of claim 4, in which said differentialdetector furthermore includes an auto regulation counter that isactuated in order to decrement from a predetermined capacitance down to0 or to increment from 0 up to said predetermined capacitance when, withthe value of the output pulses (S4) from said comparator being equal to0, the value of the digital signal output by said analogue/digitalconverter is not equal to 0 and its width (TS2) is smaller than saidfirst minimum reference (REF1).
 6. The device of claim 3, in which saiddifferential detector furthermore includes an autoregulation counterthat is actuated by said microprocessor in order to decrement from apredetermined capacitance down to 0 or increment said predeterminedcapacitance from 0 (counter=0) when, the value of the output pulses (S4)from said comparator being different from 0, their width (TS4) issmaller than said second predetermined minimum reference (REF2).
 7. Thedevice of claim 5, in which said counter is not actuated fordecrementing or incrementing (counter=0) when the value of the outputpulses (S4) from said comparator is equal to 0 and the value of thedigital signal (S2) output by said analogue/digital converter is equalto
 0. 8. The device of claim 7, in which, when it turns out that saidautoregulation counter has finished decrementing or incrementing(counter=1), said sensitivity level (NS) is decremented by 1 by saidmicroprocessor and said counter is again actuated for decrementing orincrementing (counter=0).
 9. The device of claim 1, further comprisingan autocalibration counter that is actuated by said microprocessor inorder to decrement from a specified capacitance down to 0 or toincrement from 0 up to said capacitance (counter=0), an autocalibrationof the device being carried out when said counter has finisheddecrementing or incrementing (counter=1).
 10. The device of claim 9, inwhich the value of said signal (S3) delivered to the negative (−) inputof said comparator results from the charging of a capacitor by pulsesdelivered by said microprocessor during a time interval N, theautocalibration consisting in incrementing the value of N by 1 over aspecified period as long as the values of the digital signal (S2) outputby said analogue/digital converter and of the output pulses (S4) fromsaid comparator are equal to
 0. 11. The device of claim 10, in which thevalue of N is decremented by 5 when the value of the digital signal (S2)output by said analogue/digital converter is equal to 0 while the valueof the output pulses (S4) from said comparator is different from
 0. 12.The device of claim 1, wherein said predetermined frequencies F1 and F2are equal to 0.8 Hz and 1.2 Hz respectively.